Electron microscopy, whether it be SEM or TEM/STEM, is a powerful tool in materials science and is routinely used in the assessment of feature size and materials microstructure. Electron microscopy is, however, capable of far more than this. This is particularly the case for photovoltaic materials where it is the electrical properties which control the eventual performance. In Durham we have been using cathodoluminescence in the SEM, combined with advanced TEM characterisation, to understand the electrical properties of CdTe and Cu2ZnSnS4 (CZTS) thin film photovoltaic materials.
Cathodoluminescence, the emission of photons due to electron beam irradiation, is a useful tool for phase analysis and band gap measurement but it is also a useful quantitative tool. In semiconductor materials the CL signal can be used to measure the electrical activity of defects including grain and phase boundaries. The different electrical activities of twin and grain boundaries in CdTe are demonstrated to be measurable in polycrystalline material for the first time and hyperspectral imaging has been performed to show how the emission spectrum varies over the thickness of the coating. In CZTS, cathodoluminescence measurements have been used to show the different electrical activities of CZTS phase boundaries with respect to ZnS, SnS and CuSnS3; these measurements support previous experimental observation regarding the detrimental effects of these phases in devices. Further investigation of the CZTS/ZnS phase boundary with TEM shows that the low electrical activity is due to the semi-coherence of this interface. The implications of these findings for further device development are discussed.